Field of the Invention
The present invention relates to an exposure control apparatus, a control method therefor, a storage medium, and an image pickup apparatus, and in particular to control of exposure when taking moving images.
Description of the Related Art
In general, when a still image is taken using an image pickup apparatus such as a digital camera, a shooting screen is divided into a plurality of areas to perform photometry. Then, evaluative photometry which divides a shooting screen into a plurality of areas and determines exposure according to a balance between light areas and dark areas in the plurality of areas, spot photometry which measures only a predetermined small area, or the like is performed. In any of these photometric modes, to determine areas to be measured or determine weights for split areas, positional information on a focus detection area that is to be brought into focus is used.
On the other hand, in photometry performed when moving images are taken, exposure is controlled with importance put on exposure stability as for movement of a subject and changing of a composition. Accordingly, some image pickup apparatuses have, as its dedicated photometric mode, center-weighted average photometry which averagely measures a relatively wide range with a weight put on a central part of a screen.
Further, in recent years, a method that detects a person's face from a taken image and tracks the same has been used for image pickup apparatuses. Also, there is an image pickup apparatus which, in a case where a person's face is detected when taking moving images, provides control to switch the photometric mode from center-weighted average photometry to face evaluative photometry with a weight given to split areas (photometric split zones) of a face portion.
In this image pickup apparatus, when no person's face is detected, exposure is controlled using center-weighted average photometry. In this case, for example, in a composition with a central part of a screen being bright and a slightly-backlit person lying at an edge of the screen, correct exposure is obtained with blown-out highlights suppressed in the central part of the screen, whereas exposure for a face of the person is somewhat dark.
In addition, when a state in which whether or not it is possible to detect a face of a person is not certain continues, that is, a state in which face detection is possible and a state in which face detection is impossible occur alternately, the image pickup apparatus described above intermittently switches between face evaluative photometry and center-weighted average photometry. As a result, exposure intermittently changes, causing the brightness of moving images to instability.
There is known an image pickup apparatus which to prevent the above described switching of photometric modes, performs processing to cope with chattering of face detection results, that is, refers to a plurality of face detection results to determine a photometric mode (see Japanese Laid-Open Patent Publication (Kokai) No. 2010-41399).
However, trying to prevent instability of face detection results by performing processing to cope with chattering presents a problem described below.
When an object cuts across a screen during taking of moving images in a composition with a person in the screen, there is a delay from when the object starts to cut across in front of the person to when a judgment result indicative of face detection impossible is obtained because a plurality of face detection results are referred to. For this reason, from when the object starts to cut across in front of the person to when a judgment result indicative of face detection impossible is obtained, face evaluative photometry is performed with a brightness of the object, which has cut across in front of the person, regarded as a brightness of the person's face, and exposure control is influenced by the object.
For example, when an object that has cut across a screen is black, a face that has been correctly exposed may be temporarily overexposed to cause instability of exposure. Particularly in a shooting scene with two persons, a black object cutting across a main subject (main face) leads to incorrect exposure for the other subject.